1. Field of the Invention
The present invention relates to a heater control device for supplying a current to a heater to heat a load.
2. Description of the Related Art
In an image forming apparatus, such as a copying machine and a printer, using an electrophotographic process, an electrostatic latent image formed on a photoconductive drum (rotary photoconductor body) is converted into a visible image (hereinafter referred to as a toner image) by making a development agent (hereinafter referred to as toner) to adhere thereto through a development unit. The toner image is transferred to a recording paper sheet through a transfer unit, and the toner image is then fixed to the recording paper sheet through a fixing unit to form a permanent image thereon.
The fixing unit typically employs a thermal fusing fixing method, in which the toner is fused by thermal energy from a fixing roller heated by a heater as a heat source so that the toner is fused into the fiber of the recording paper sheet under the pressure of the fixing roller.
FIG. 15 is a block diagram showing a major portion of a conventional image forming apparatus which controls power supplying to a fixing heater which is used as a heat source in a fixing unit of thermal fusing fixing type.
Referring to FIG. 15, an alternating current power is supplied across input terminals 1 and 2 connected to an alternating current utility power line. Connected between the input terminals 1 and 2 is a series network, consisting of a fixing heater 3 and a switching element 4, to which the alternating current utility power is supplied. The fixing heater 3 is housed in an unshown fixing roller and is extended in the axial direction of the fixing roller. A halogen lamp with a power rating of several hundred W to 1 kW and with a resistance of a positive temperature coefficient is typically used for the heater. A solid-state relay (SSR) or an insulated switch circuit constructed of a phototriac and a triac may be used for the switching element 4.
A zero crossing detector circuit 6, connected across the series circuit, detects the zero crossings of the alternating current utility power supplied to the series circuit between the input terminals 1 and 2. A temperature sensor 5 is arranged in the close vicinity of the surface of the fixing roller, and is typically a thermistor with an impedance having a known temperature coefficient. This arrangement allows the temperature of the surface of the fixing roller to be constantly detected, and a detected temperature signal is output to a temperature adjusting circuit 7.
The temperature adjusting circuit 7 controls the switching element 4 for switching in response to the detected temperature signal, thereby controlling the on/off timings of the fixing heater 3. The temperature adjusting circuit 7 outputs heater on/off signals for temperature control to a drive pulse generator 12 to keep the temperature of the surface of the fixing roller to within a predetermined temperature control range. More specifically, the temperature adjusting circuit 7 outputs an off signal when the surface temperature of the fixing roller rises to the upper limit of the predetermined temperature control range and outputs an on signal when the surface temperature of the fixing roller drops to the lower limit of the predetermined temperature control range.
Receiving the heater on/off signals and the output of the zero crossing detector circuit 6, the drive pulse generator 12 outputs a drive pulse to the switching element 4 to control the surface temperature of the fixing roller to within the predetermined temperature control range.
The operation of the circuit arrangement shown in FIG. 15 is now discussed.
When the alternating current utility power is supplied across the input terminals 1 and 2, an unshown power supply circuit rectifies it to a direct current power to energize the above-described circuits 4, 6, 7 and 12. The temperature sensor 5 detects the surface temperature of the fixing roller and outputs the detected temperature signal to the temperature adjusting circuit 7. The temperature adjusting circuit 7 outputs the on signal to the drive pulse generator 12 when the detected surface temperature of the fixing roller drops below the lower limit of the predetermined temperature control range and outputs the off signal to the drive pulse generator 12 when the surface temperature of the fixing roller gradually rises and reaches the upper limit of the predetermined temperature control range.
The zero crossing detector circuit 6 continuously monitors the alternating current utility power to detect its zero crossings, and outputs the zero crossing signal to the drive pulse generator 12.
In response to the on signal and off signal coming in from the temperature adjusting circuit 7, the drive pulse generator 12 generates and outputs the drive pulse to the switching element 4 in synchronization with the zero crossing signal output by the zero crossing detector circuit 6, thereby causing switching element 4 to switch on and off. The switching element 4 is thus controlled. The switching action of the switching element 4 controls intermittent conduction timings of the fixing heater 3.
The current flowing from the alternating current utility power line to the fixing heater 3 is controlled such that it always starts to flow in synchronization with the zero crossing of the alternating current utility power. The fixing roller is controlled such that its surface temperature is kept to within a predetermined temperature control range.
FIGS. 16A-16C are waveform diagrams showing the relationship between the current flowing through the fixing heater 3 and the drive pulse.
FIG. 16A shows a waveform Lin2 flowing through the fixing heater 3, where F represents one period of the alternating current frequency. FIG. 16B shows the drive pulse, in which the switching element 4 remains on during a high level (ton) and remains off during a low level (toff). FIG. 16C shows the root-mean-square value of Lin2rms into which the current wave Lin2 is converted every half period of the alternating current frequency.
Since the switching element 4 remains off during the toff period, the fixing heater 3 is not powered, with no current supplied. The fixing heater 3 is housed in the fixing roller. The fixing roller has a larger thermal capacity while the fixing heater 3 has a smaller one. For this reason, the surface temperature of the fixing roller slowly drops while the temperature of the fixing heater 3 rapidly drops. The fixing heater 3 drops in temperature during the toff period because it generates no heat and its resistance is extremely small during this period.
The drive pulse is now driven high to a high level with the surface temperature of the fixing roller lowered. The fixing heater 3 is supplied with the alternating current utility power. This means that the utility power is fed to an extremely low resistance. At the start of power feeding, a very large rush current flows in contrast to the stationary state shown in FIG. 16A. As the resistance increases with the fixing heater 3 rising in temperature during the ton period, the current stabilizes to the stationary state shown in FIG. 16A.
The root-mean-square value Lin2rms converted from the current waveform Lin2 changes as shown in FIG. 16C. The root-mean-square value RS.sub.3 corresponding to the rush current, in contrast to a root-mean-square value ST converted during the stationary state, is greatly dependent on the temperature control range of the fixing roller (the range from a lower temperature limit above which the fixing heater 3 is powered to an upper temperature limit above which power is removed from the fixing heater 3). The root-mean-square value is greatly dependent on the lengths of the high-level period and low-level period of the drive pulse. Specifically, a shorter low-level period results in a lower rush current peak value RS.sub.3 and a longer low-level period results in a higher rush current peak value RS.sub.3. When the low-level period gets longer than a predetermined duration, the fixing heater 3 fully drops, saturating the value RS.sub.3. In the example shown in FIGS. 16A-16C, the rush current peak value RS.sub.3 flowing through the fixing heater 3 at the moment the switching element 4 is transitioned from off to on becomes several times as great as the stationary root-mean-square value ST.
In the image forming apparatus such as a printer or copying machine having such a heater control circuit therewithin, the peak current represented by RS.sub.3 flows at the moment the supplying of the alternating current utility power starts. Unless the impedance of the interior wiring for feeding the alternating current utility power is sufficiently low, a large voltage drop instantaneously takes place through the impedance in the alternating current utility power supply. This possibly adversely affects other apparatuses that share the same line for the alternating current utility power. As one of such examples, an instantaneous voltage drop causes a flickering in which illuminance level of lighting equipment drops momentarily. To prevent such a voltage drop, the impedance of the power line may be lowered or a complex and costly circuit arrangement may be used.
Although the use of two heaters for the fixing unit is contemplated, such a fixing unit equally suffers the same problem.